1. The Field of the Invention
The present invention generally relates to an optical device for attenuating electromagnetic radiation which propagates along a waveguide. More particularly the invention relates to an optical device for the variable attenuation of light within optical fibers.
2. The Relevant Technology
With the advances in technology in recent years, numerous attempts have been made to increase the flow of information between government agencies, private businesses and in general, from individual to individual. The need for information is ever increasing and hence many different types of transmission lines have been developed, such as optical fibers.
An optical fiber is a type of waveguide which allows the transmission of light along its length through the property of total internal reflection. The light beam or ray carries information as it propagates along the core of the optical fiber. The majority of the light beam remains within the core material since the core has a refractive index which is higher than the cladding material which surrounds the central core.
Optical fibers may be used to transmit information over both short or long distances, however, the optical properties of the optical fiber can be engineered based on the intended use. Furthermore, in optical communications, losses occur in light intensity and power along the length of the optical fiber. These losses are caused by absorption, scattering or geometric effects associated with the manufacture and use of the optical fiber. For example, light intensity is reduced by impurities contained within the optical fiber. Light is also absorbed due to splicing or connecting optical fibers and/or microscopic bending formed during fiber manufacture. Due to the variation in light intensity, the operation of the final optical components vary greatly, resulting in degradation of performance and efficiency. Optical attenuators provide a variety of useful functions to solve the above problems.
Optical attenuators perform numerous tasks associated with optical fiber communication. One function of an attenuator is to reduce the intensity of an optical signal which enters a photosensitive component. Photosensitive components are affected by variations in light intensity. Therefore, an attenuator causes the light intensity to be within the dynamic range of the photosensitive components. By using an attenuator, damage to the component is precluded. Additionally, the component does not become insensitive to small changes in the optical signal.
In other applications, attenuators serve as noise discriminators by reducing the intensity of spurious signals received by the optical device to a level below the device""s response threshold. Moreover, optical attenuators are used to reduce the power of optical signals from an input fiber to an output fiber, and especially to balance optical power between several lines of an optical system. Many optical attenuators are also capable of actively attenuating an optical signal. Variable attenuators are required in some applications where different optical components require dissimilar incident optical signals, and hence variable sensitivities and saturation points. A fixed (i.e., passive) attenuation device is impractical for this purpose.
Attenuators serve to maintain the light level at a constant to compensate for component aging i.e., loss of efficiency in fiber amplifiers and reduced laser output from source, and changing absorption in optical waveguides. Variable attenuators serve to control feedback in optical amplifier control loops to maintain a constant output (e.g., as an automatic gain control element (AGC)).
Because of the variety of operations in which optical attenuators can be used, numerous types of optical attenuators have been developed. For example, U.S. Pat. No. 5,276,747 to Pan describes a liquid crystal optical attenuator device. This device uses a liquid crystal element which is manipulated to variably attenuate an incident light ray. The light travels through the liquid crystal element and is attenuated as the liquid crystal molecules are reoriented. Unfortunately, liquid crystals are temperature sensitive and hence as the temperature of the surrounding air varies, the optical properties of the liquid crystal also change. This results in inefficient transmission and inaccurate attenuation by the device.
Another group of optical attenuators are those which are mechanically activated. These vary in the method by which attenuation is caused. For example, one type of optical attenuator uses a rotatable filter placed between an input fiber and an output fiber. As the filter is rotated, the level of attenuation is varied. Another mechanical optical attenuator involves moving the ends of the input and output fibers such that the axis of the fibers are no longer aligned. By changing the angular misalignment between the input fiber and the output fiber the quantity of light transmitted along the output fiber is varied, hence variable attenuation occurs. In yet another mechanical attenuator the mechanism bends the optical fiber around a tapered or cylindrical element. As the element is rotated, different bending radii occur along the length of the optical fiber, thereby inducing radiation losses and hence variable attenuation. An alternative method of mechanically varying the attenuation of an optical signal is to use a moveable reflector, such as a mirror. As the mirror is moved, the angle at which the incident ray is reflected varies, thereby varying the light directed towards the output optical fiber, and causing attenuation.
Unfortunately, mechanical optical attenuators have the disadvantage that they use mechanical elements to either move the optical fiber or move elements which effect the optical fiber. These mechanical elements wear and eventually fail, resulting in a loss of attenuation. Furthermore, mechanical attenuators are relatively slow because of the mechanical movement compared to electrical, thermal, electromagnetic or similar movement found in other optical attenuators.
A hybrid type attenuator uses both mechanical movement of the optical fiber and modification of optical characteristics of the optical fiber. For example, U.S. Pat. No. 5,694,512 to Gonthier et al. discloses an optical attenuator which uses a tapered optical fiber. The fiber is heated and shaped to form the taper which acts to attenuate a given wavelength of light. This attenuator then may become a variable attenuator by applying a force to the optical fiber to bend the optical fiber and cause additional transmission losses. Numerous tapers and numerous bending elements may be used to vary the attenuation. Unfortunately, the movement of the optical fiber and/or bending of the optical fiber reduces the tensile strength of the optical fiber, thereby shortening the life of the fiber and hence the optical system. Furthermore, the stresses which are applied to the specific locations on the optical fiber can cause additional temperature stresses or temperature gradients along the optical fiber. The optical properties of the optical fiber are thereby changed, reducing control and effectiveness of the attenuator.
Another type of attenuator uses a series of Bragg gratings to attenuate various wavelengths of light. A Bragg grating acts as a filter to transmit and reflect different wavelengths of incident light. The Bragg grating is formed by exposing the core of the optical fiber to a beam of ultraviolet (UV) light. The UV light produces a permanent refractive index variation in the core of the optical fiber which reflects and transmits different wavelengths of incident light. Unfortunately, Bragg gratings are fixed in the optical fiber. The molecular structure of the fiber is modified to prevent the passage of a required wavelength of light. Once the optical fiber is modified, it cannot be reversed. Furthermore, one Bragg grating will only attenuate a given wavelength and cannot be varied. To form a variable attenuator using Bragg gratings it is necessary to direct the light or optical signal through multiple fibers and multiple Bragg gratings. Consequently, there is an increase in size and cost associated with this type of attenuator.
Finally, another type of attenuator uses an electrochromic element to cause attenuation of incident light within a waveguide. In U.S. Pat. No. 4,245,883 to Johnson et al., a number of waveguides are completely or partially embedded in or supported by a substrate which also serves as an optical cladding. The waveguides are bonded to an electrochromic body which allows for the attenuation of the light incident within the waveguides. As the light travels within the fixed waveguides, the light comes into contact with the electrochromic body. As a current is applied between a pair of conductor layers, the optical characteristics of the electrochromic body vary and cause attenuation of the incident light. Unfortunately, the light within the waveguide travels parallel to the conductors and passes through the entire width of the electrochromic body. The electrochromic layer is therefore relatively thick in relation to the propagating light. Additionally, the waveguides are fixed to the substrate which reduces the effective use of the attenuator since the substrate is formed from a solid member.
It would therefore be an advance to provide a variable optical attenuator which overcomes the above problems.
It is an object of the present invention to provide an optical attenuator having no moving parts and which has broad band capability.
A further object of the present invention is to provide an optical attenuator which is compact in size and relatively inexpensive.
It is another object of the present invention to provide an optical attenuator which is wavelength independent and non-polarizing.
Still yet another object of the present invention is to provide an optical attenuator which is polarization insensitive while retaining a broad bandwidth.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein, a variable optical attenuator device is provided for modulating an optical signal. The attenuator device includes a variable attenuation assembly with an electrochromic structure interposed between a first electrode and a second electrode. The electrochromic structure is configured to change its optical characteristics from a bleached off state to a colored active state under the influence of an electrical potential applied to the first and second electrodes to thereby modulate the optical signal. The optical attenuator device includes at least one lens attached to the variable attenuation assembly. The lens cooperates with the variable attenuation assembly to direct the optical signal towards the electrochromic structure. Waveguides such as optical fibers define ports at the outer endface of the lens for the optical signal.
The first electrode in the variable attenuation assembly is configured to be transmissive to an optical signal from an input waveguide attached to the lens. In one embodiment, the second electrode is configured to reflect the optical signal back towards the lens and into an output waveguide attached to the lens. In an alternative embodiment, an additional lens is attached to the variable attenuation assembly on the second electrode, which is configured to be transmissive to the optical signal. An output waveguide is attached to the additional lens for receiving the optical signal transmitted through the second electrode.
In other embodiments of the invention, multichannel variable optical attenuators are provided by attaching multiple lenses and/or waveguides to the variable attenuation assembly.
In a method for attenuating an optical signal according to the present invention, a variable attenuation assembly is provided including an electrochromic structure interposed between a first electrode and a second electrode, with the first electrode being transparent to an optical signal. An optical signal is directed to the variable attenuation assembly and is allowed to pass into and out of the variable attenuation assembly with substantially no loss in signal strength during an inactive state. An electrical potential is applied to the first and second electrodes such that the electrochromic structure attenuates the optical signal to a desired level during an active state.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or maybe learned by the practice of the invention as set forth hereinafter.